It has become standard practice to plan wells and analyze bit performance by using log-based rock strength analysis and/or specific energy theory. The most widely used characterization of rock strength is unconfined compressive strength (UCS), but this is somewhat problematic because the apparent strength of the rock to the bit is typically different than UCS. Specific energy theory has been used for bit performance assessment for years. One of the challenges of application of the specific energy theory, however, is uncertainty or lack of consistency in reasonable values for input variables to be used in specific energy based equations.
The present invention addresses the need to provide reasonable values for the input variables used to predict rate of penetration and reactive torque of a drill bit using specific energy theory